1. Field of the Invention
The present invention relates to a flip-chip mounted surface acoustic wave (SAW) device and a method of producing the same.
2. Description of Related Art
Conventionally, surface-acoustic-wave (SAW) devices are fabricated through a series of steps of adhesively fixing a SAW chip on the inside bottom surface of a package and then interconnecting the bonding pads formed in the package and the bonding pads on the surface of the SAW chip through the wire bonding. However, these wire-bonded-type SAW devices have a problem in that the adhesive agent used for fixing the SAW chip leads to variations in characteristic after fabrication. In contrast, flip-chip mounted-type SAW devices are fabricated through the following steps. That is, bumps made of a conductor such as gold are placed on conductive pads of a SAW chip. The SAW chip is placed on the bottom surface inside the package, with the bump formed surface (electrode formed surface) facing the bottom surface inside the package (die attachment surface). The intermediate structure is subjected to heating, pressure, or ultrasonics. Thus, the conductive pads on the die attachment surface and those on the electrode formed surface are electrically connected and mechanically fixed by the bumps. This process does not require any adhesive agent. Hence, the resultant SAW device has less variation in characteristics than do conventional wire-bonded-type SAW devices.
FIG. 7 illustrates the configuration of a flip-chip mounted SAW device. Referring to FIG. 7, electrodes or various conductive layers (not shown) are formed on the electrode formed surface 10a of a SAW chip 10. The electrodes may be ones that excite a surface of a Piezoelectric substrate of, e.g. quartz crystal, by applied electrical signals to generate surface acoustic waves. The electrodes may receive surface acoustic waves and produce electrical signals or may reflect surface acoustic waves in a specific direction. Moreover, conductive pads (not shown) electrically connected with the electrodes on the electrode formed surface 10a are connected to and fixed to the package 12. The package 12 has a recess to accommodate the SAW chip 10. Conductive pads (not shown) are formed on the bottom surface, or the die attachment surface 12a, so as to face bumps 14 when the chip 10 is accommodated in the recess. Footprints 12b, being conductors for external connection, are formed on the package 12. The footprints 12b are electrically connected to conductive pads on the die attachment surface 12b through conductors (not shown) penetrating the package 12.
In the fabrication of the SAW device in FIG. 7, bumps 14 made of a metal such as gold are first placed on the conductive pads formed on the electrode formed surface 1Oa. The SAW chip 10 is placed in the recess of the package 12, with the electrode formed surface 10a downward in FIG. 7. The intermediate structure is subjected to heating, pressure, or ultrasonics. Thus, the conductive pads of the SAW chip 10 are electrically connected and mechanically fixed to the conductive pads on the die attachment surface 12a via the bumps 14. Thereafter, the lid (cover) 12c seals the recess to house the SAW chip 10. The sealing portion 12d, which is a sealing member intervening between the lid 12c and the package 12, is formed by applying heat or pressure. Gold or tin, resin, soldering, glass and so on are among the substances that may be used for the sealing member.
With the package 12 connected and fixed with the bumps 14, the characteristics of the SAW device 10 can be measured with an external measuring device via the footprints 12b before sealing the opening of the package 12. One of the inventors of the present invention also invented the method disclosed in Japanese Patent Laid-Open Publication No. Hei NO. 10-145167 (hereinafter referred to as a previous proposal). According to this method, before the opening of the package 12 is covered with the lid 12c after the bump connection and fixture, focused radiation such as laser light is irradiated from above the back surface of the SAW chip 10 (from the upper orientation in FIG. 7), to adjust the characteristics of the SAW chip 10. Thus, the radiation evaporates the conductive layer or the insulating layer on the electrode formed surface 10a while the characteristics of the in-process item is being measured. Though this process, the device characteristics can be adjusted to suppress and prevent characteristic variations due to conditional deviations in fabrication.